In electric power distribution, fault interrupting devices such as reclosers and reclosing circuit breakers are equipped with a mechanism that automatically closes the fault interrupting device after it has been opened due to a fault. Fault interrupting devices are also used on overhead electric distribution systems to detect and interrupt momentary faults. Since many short-circuits on overhead lines clear themselves, a fault interrupting device improves service continuity by automatically restoring power to the line after a momentary fault.
The control system for a fault interrupting device allows a selected number of attempts to restore service after adjustable time delays. For example a recloser may have two or three relatively fast reclose operations with a few seconds or even fractions of a second delay, followed by a longer delay and one reclose. If the last attempt is not successful, the recloser locks out and may require human intervention to reset. If the fault is a permanent fault (e.g. downed wires, tree branches lying on the wires, etc.) the recloser exhausts its pre-programmed attempts to re-energize the line and remains tripped off until manually commanded to try again. About 80-90% of faults on overhead power lines are transient in nature and can be cleared by autoreclosing, resulting in increased availability of supply.
However, autoreclosing results in additional burden on power equipment in situations when the fault is permanent. Fault current magnitudes are typically much higher for permanent faults, especially for close-in faults. Reclosing into a permanent fault upsets the substation bus and adversely affects power quality on the healthy feeders. For example, in the current practice of reclosing, substation transformers are subjected to damaging short-circuit currents several times before the recloser locks out for a permanent fault. Additionally, the interrupter mechanism of reclosers/circuit breakers are subjected to several clearing cycles as well. These devices have limited life in terms of the number of faults and current magnitudes they can safely interrupt before maintenance is needed.
Some reclosing techniques involve utilizing condition-based control as opposed to more traditional time-based control typically used in practice in an attempt to avoid reclosing into permanent faults. For example, substation circuit breakers and field reclosers can be equipped with microprocessor-based electronic control devices referred to herein as Intelligent Electronic Devices (IEDs). IEDs accept analog signals from potential and current transformers, digitize the analog signals through an A/D (analog-to-digital) conversion stage, execute signal processing and analysis tasks via internal instructions, and issue control signals in response to power system operating conditions that require protection and/or control actions. All of these functions are conventionally internal to the IED.
Due to the tight integration of many functions in a physical IED, these devices are I/O (input/output) bound and/or processing power bound. I/O bound means that the IED cannot accept an arbitrary number of I/O channels beyond its specification. This limits the extended use of the IED when new needs arise over time, resulting in costly hardware change-outs in the field. Bound by processing power means that the IED processor is limited to performing tasks originally considered in the design stage, again limiting the extended use of the IED in the field. For example conventional IEDs used for protection purposes only require a low sampling rate of about 32 samples/cycle (1.92 kHz @ 60 Hz fundamental frequency) or less, that may limit the type of events such as faults and high frequency transients that can be detected and analyzed.